Queen Victoria's Bomb by Ronald Clark

The Flashman books by GM Fraser are historical novels with the titular character finding himself at the main events of 19c colonialism. They give really good flavour of how the different actors back then would have experienced the times from various levels. Plus, they're really funny and well researched. You can take your pick based on what event you want to read on. The Great Game by Peter Hopkirk (and rest of his writings) gives the story by character of those involved in the Great Game. Highly recommended if you like Kim, The Man who Would be King etc Also second Morris from yarb - they're not written as a pro-empire argument, but Jan Morris so clearly personally relishes in the period that you struggle not to get taken along with the romance of it. I'm English though so ymmv

The best book on British colonialism I've read is Jan Morris's "Pax Britannica" trilogy, which tracks the empire over the course of its life through a kaleidoscope of vignettes from across the years and continents. I don't think it's pro-Brit or pro-Colonialism, but it's not anti- those things either, and I suspect many readers today would (perhaps wrongly) see it as a celebration of empire. So yeah I think it could be construed that way. In any case it's a superb, vivid, history, written with boundless curiosity and compassion.